The invention relates to a method and to a device for controlling a lift cylinder of the class specified in claim 1 or 2.
Hydraulic lift cylinders of working machines can be lowered without additional energy input when they are under load, in which case it is standard practice to ensure oil filling both on the piston side, or in other words in the pressure cylinder space, and on the rod side, or in other words in the suction cylinder space.
It is known that lift cylinders of working machines can be lowered by directing the pressurized oil forced out by the load to a tank by means of a control unit or by natural flow, while the hydraulic pump delivers hydraulic oil to the suction side of the cylinder in order to prevent cavitation. This procedure suffers from the disadvantage that additional energy must be expended for filling the suction side.
In another solution, the suction side of the cylinder is connected to the tank, so that the oil is sucked out of the tank by the suction side, thus causing the disadvantage that line resistances between the cylinder and tank must be overcome, possibly leading to incomplete filling of the cylinder. This is countered by flow restrictors in the tank return line, but then heat is generated and must be removed by cooling.
For reasons of energy economy, the challenge comprises being able to lower the cylinder in controlled manner without input of energy, or in other words to use the energy of the hydraulic oil flowing out of the lift side for filling the suction side without the need for flow restrictors. For this purpose it is known that the pressurized outflowing hydraulic oil can be directed by a control element to the suction side of the cylinder, in which case a separate lowering valve, for example, is provided between cylinder and main control valve and, during the lowering process, is actuated instead of the main slide valve, in order to direct the necessary partial quantity to the suction side of the cylinder, while the remaining quantity can bypass the main slide valve and flow off into the tank.
There is also known a further solution in which the main control slide valve for the lift cylinder can be designed as a hollow piston, which in lowering position establishes a transfer line between the pressure side and the suction side of the cylinder, in which case a check valve is disposed in the hollow piston to disconnect the connection during normal operation. The production of such a hollow piston as a control piston in the main control slide valve is expensive, and the corresponding circuit for lowering under gravity has the disadvantage that the lift cylinder cannot apply any additional force in lowering direction. In order to achieve an effect in lowering direction also, the circuit for lowering under gravity must be disconnected.
Compared with the known solutions described in the foregoing, the object of the invention is to provide a procedure and a device with which, by adjustment of the control slide valve, the suction space of the cylinder is adequately filled under all pressure conditions during the lowering process, a changeover to normal operation of the cylinder taking place as soon as an additional cylinder force in lowering direction is needed.
According to the invention, this object is achieved with a method of the type cited in the introduction by the fact that
a distribution channel in the control element is pressurized by the hydraulic pump, whereupon the pressure cylinder space or the suction cylinder space is connected via a control piston,
at a first predetermined pressure produced in the cylinder space by the external force/load (P), the cylinder space is connected via a check valve to this distribution channel and the feed flow from the pump to the distribution channel is stopped by a stop valve that can be influenced by a pressure sensor, whereupon the suction cylinder space is connected to the channel, and
when a second predetermined pressure is reached, the check valve closes and the stop valve is opened, thus allowing feed flow from the hydraulic pump to the distribution channel for application of an additional force in the suction cylinder space.
From the viewpoint of device design, the object cited in the foregoing is achieved by the fact that there is provided, in the control element, a distribution channel that can be pressurized by the hydraulic pump, the distribution channel having two outlet channels that can be connected and disconnected via a control piston, the outlet channels having connecting lines to the pressure cylinder space and suction cylinder space respectively of the lift cylinder, a check valve being provided between the channel and the distribution channel, a stop valve that influences the pump feed flow being provided in the distribution channel, a pressure sensor being provided between channel and pressure cylinder space and a switch that can be activated thereby being provided for actuation of a hydraulic valve for actuation of the stop valve.
By means of the inventive procedure it is evidently possible, via the control element, to direct the oil arriving from the pressure space of the cylinder to the suction side, in which case pressure monitoring in the pressure space as well as appropriate adjustment of the check valve and switching of the position of the control valve ensures lowering without application of an external force.
In this connection it is particularly expedient that, in the case of complete lowering of the working machine by the hydraulic cylinder, for example, the pressure in the pressure space of the cylinder can be reduced or completely adapted to the system pressure, by the fact that the check valve closes and the stop valve can be opened if necessary. This is also possible without problems for the case of lowering under pressure, in which case, via the stop valve, pressurization that can act in lowering direction is made possible by the pump. If the control piston in the control element is switched, the pump can then act in lifting direction by pressurizing the pressure space.
If, for example, a pressure of greater than 20 bar due to the external force is signaled to the pressure switch, the control valve is switched to such a position that the hydraulic fluid closes the stop valve in the pump distribution channel and thus stops the feed flow of hydraulic oil from the pump, so that the hydraulic oil flows out of the pressure cylinder space of the cylinder via the check valve into the suction cylinder space, while excess oil can be directed back into the tank if necessary.
If, for example, the pressure drops in response to a decrease of external load in the pressure cylinder space, the check valve closes. A control signal is automatically transmitted via the pressure sensor to the relay valve, whose position is switched, thus depressurizing the stop valve, so that this automatically opens and allows feed flow of hydraulic oil via the pump to the suction side of the cylinder, thus permitting pressure to be exerted on the cylinder in lowering direction.
If lifting is required, the control side of the piston is depressurized, so that this spring-loaded piston extends, thus opening feed flow of hydraulic oil to the pressure side of the piston.
Further embodiments of the invention are specified in the dependent claims.
In a particularly advantageous embodiment, for example, control grooves that enable flow to and from both the lift and suction sides of the cylinder are provided at the control edges of the control piston, the grooves on the lift and suction sides corresponding to the ratio of areas of the lift and suction sides of the cylinder.
An expedient and compact construction is also achieved by forming the check valve as an integral unit with a secondary pressure limiting valve.
The invention also makes it possible to use two pumps, in which case it is expedient to provide at least one controlled stcp valve and one controlled check valve in the system.